This invention relates generally to improvements in mills for pulverizing particulate material and classifying the resulting particles.
Pulverizing and classification of dry materials is practiced today using hammer-screen mills, impact attrition mills, ball mills, pin mills and others outfitted with internal classifiers that separate the coarse and the fine particle fractions. The air classifying mill is popular equipment for pulverizing and separating particulate material (e.g., chemicals, minerals, fertilizers, foodstuffs, and powder coatings) into selectively sized particles. In general, air classifying mills can be classified into two types; single- and dual-drive.
Dual-drive mills can pulverize a wide variety of materials and allow the operator to control the particle size simply by drive speed adjustment. Typically, dual-drive air classifying mills comprise a housing containing a pulverizing disk and a particle classifier, wherein each of the pulverizing disk and the classifier are controlled by a separate drive. One drive adjusts the classifier speed to control the particle size while the second drive controls the pulverizing disk speed, which is adjusted depending on the hardness and toughness of the particulate material. Because the speeds of the classifier and the pulverizing disk are each controlled by a separate drive, they can be adjusted independently. A disadvantages of dual-drive mills is that they are mechanically complicated and thus more expensive and more prone to mechanical failure than mills of simpler construction.
Single-drive mills are of more simple construction, less expensive, and more rugged because both the pulverizing disk and particle classifier are driven by the same drive. As such, the operator has limited control over particle size, and versatility with respect to material hardness is very limited. Consequently, there is less control of particle size and the type of particulate material that can be pulverized is relatively limited.
A need exists for an economically priced and rugged single-drive particulate material pulverizing and classifying mill capable of conveniently producing selectively sized particles from a wide variety of materials.
The present invention provides a mill that satisfies this need. The invention provides a cost efficient and rugged single-drive air classifying mill of simple design capable of operating at high efficiency and able to produce a wide range of particle sized for a wide range of materials.
In one embodiment, a mill is provided for pulverizing and classifying particulate material comprising: a housing having an air intake, a particulate-material inlet, a particle outlet, and a particulate material pulverizing and classifying assembly for receiving the particulate material from the inlet, pulverizing the particulate material into pulverized particles, and separating the pulverized particles by a size of the pulverized particles. The pulverizing and classifying assembly is rotatably mounted on a drive shaft and positioned within the housing. Advantageously, the mill has a particle-removal conduit connected to the particle outlet and extending into the assembly for removal of the pulverized particles. Preferably, the housing is provided with an inner liner having a selectively shaped pattern to assist in pulverizing the particulate material.
In another embodiment, the mill of the invention has a particulate material pulverizing and classifying assembly comprising a pulverizing disk rotatably supported by the drive shaft, a classifier supported by the pulverizing disk and having first and second ends. The second end is adjacent to the housing while the first end adjacent to the pulverizing disk. A pulverizing domain exists between the pulverizing disk and the housing and the particle-removal conduit extends into the second end of the classifier for separating the pulverized particles according to size.
Also, it is preferred that the extension of the particle-removal conduit is adjustable to control the size of the pulverized particles accepted by the classifier and thereafter directed to the particle outlet for collection.
Preferably, the second end of the classifier fits at least partially within a groove in the housing to prevent the pulverized particles from passing around the second end of the classifier. It is also preferable that the mill further comprise a shroud oriented parallel to the drive shaft and surrounding at least a portion of the assembly to separate the pulverizing domain from the classifying zone and to assist in directing the pulverized particles to the classifier.
In another embodiment, the mill of the invention comprises a classifier wherein the first end has a diameter that is different than that of the second end to create a tangential air velocity differential between the first end and the second end to assist in the separation of the pulverized particles. In one embodiment, the diameter of the first end of the classifier is larger than the diameter of the second end to provide a higher tangential air velocity at the first end. In another embodiment, the diameter of the first end of the classifier is smaller than the diameter of the second end to provide a higher tangential air velocity at the second end. Preferably, the classifier is in the shape of a cone.